This is needed on focal if a client is to be able to access snapshots over NFS.
From the docs I don't see why we didn't also need this option on xenial,
but empirically, we need it on focal. (e.g. RT-207229)

The generation of the command to unexport NFS filesystems could generate
an invalid command. Leading spaces were not being stripped, and in cases
where there is more than one backup target for a machine we need to
unexport every target. Because we also had 'set -e' in operation at this
point, the script would fail there and never clean up the moved ZFS. I
don't mind if we fail to unexport; if that's subsequently a problem for
removing the ZFS then the script will fail at that point.

This change makes the script generate better exportfs -u commands and
not exit if they fail.

The code used to open a database connection for each thread and leave
them open for as long as the scheduler ran. This worked reasonably well
until we moved to PostgreSQL 13 on Focal, although the scheduler would
fail if the database was restarted because there was no logic to
reconnect after a connection dropped.

On Focal/PG13 the connection for the 'cron' thread steadily consumes
memory until it has exhausted everything in the machine. This appears to
be a Postgres change rather than a Perl DBI change: the problem can be
reproduced by sitting in psql and running 'select * from backup_queue'
repeatedly. Once or twice a minute an instance of this query will cause
the connection to consume another MB of RAM which is not released until
the database connection is closed. The cron thread runs that query every
two seconds. My guess is it's something peculiar about the view that
query selects from - the time interval thing is interesting.
This needs more investigation.

But in the meantime I'd like to have backup servers that don't endlessly
gobble RAM, so this change makes the threads connect to the database
only when they need to, and closes the connection afterwards. This
should also make things work better over database restarts but that's
not been carefully tested.

As of focal a bunch of top-level dirs are symlinks (eg /lib -> /usr/lib) but
the deb packages still deploy files to the symlink rather than the real dir.
Thus if we just take the contents of all the *.list files we end up not
excluding lots of files that are in fact provided by debs

This package depends on a postgres server being available, but annoyingly we
can't use the 'postgresql' metapackage in the dependencies because on Ubuntu
that depends on the specific distro-provided version, which usually isn't the
one we want. So we have to add our supported Postgres versions one by one.

Can't say "keys $foo", must say "keys %{$foo}" to get the keys of a hash
pointed to by a hash reference $foo. This broke with the perl in focal.

The default prepare script now uses appropriate options if it detects Postgres
12 or higher. The error handling still needs work though.

Update prepare script for newer Postgres

Looks like Subiquity sets up a swapfile at /swap.img on Ubuntu 20.04 by default
and we certainly don't want to back that up.

We don't need these backed up, and having them in the list leads to
an error due to not having show_compatibility_56 enabled

Improve pre error handling

See merge request !1

NB this is so we can put things in LOGFILE at earlier points
than the script does at present

We capture the error code at a few points in the script, and then (intend to)
log something about what went wrong. But if we set -e we immediately bail
before logging anything useful!

Resolves #1

Resolves #3

On a new machine, there's nothing to remove and so rm will return
an error. I'd like our prepare scripts not to return errors
unnecessarily!

changed maintainer
mention name of git repository since it doesn't match package name

This repo used to have a non-standard build procedure. Now it uses
our usual makedeb process so we don't need to add extra documentation
here.

We now e.g. have a cron job on cerebro-backup which calls
these scripts, where /sbin is not on the $PATH.

For cerebro-backup where we do backups by the directory rather than the
whole server. We need to have one machine being backed up
(cerebro-filestore) but a task per directory.

NB arguably we "should" be doing this via debian/rules and
calling dh_fixperms. Doing that is left as an exercise for
whoever volunteers to refactor the way we build all of our
local debs!

The new-backup-rsnapshot script understands a 'postgres' argument, but
this set up a postgres backup in an old style that we no longer use.
This change updates it to do some of the work of setting up a new style
postgres backup and tell the user what else they might need to edit to
make it go; it varies quite a lot depending on server.

Let's not worry if we haven't backed up a machine that has been offline for 3 months (instead of 6 months)